This invention relates to ring laser gyro path length control mechanisms and more particularly to such a mechanism employing the diametrical contraction of piezoelectric discs to effect mirror motion.
The optical path length in a ring laser gyro must be accurately controlled to insure acceptable gyro performance. Path length is commonly controlled utilizing piezoelectric devices which adjust the location of a movable mirror in the ring laser gyro cavity. In known configurations, piezoelectric discs are stacked and mirror motion is effected by changes in the thickness of the piezoelectric discs. When a voltage is applied to such as disc, its thickness increases. The magnitude and polarity of the voltage applied to such discs is derived from the laser output to maintain optimum laser gain and frequency. The path length control mechanism is required to compensate for dimensional changes in the ring laser gyro block and mirrors due, for example, to temperature variations or instabilities in the materials. In a typical application, the maximum distance over which the mirror must be adjusted is about 100 micro-inches. The required accuracy and resolution of mirror motion is typically less than 1 micro-inch.
As the mirror moves to control path length, it is necessary that the plane formed by the laser beams remain essentially invariant in its location with respect to the reflective surfaces of the mirror, the aperture in the path of the laser beams, and the laser electrodes because any change in the location of this plane will also affect the laser gain and the laser bias conditions. Deformation of the laser block due to accelerations or thermal variation in a direction transverse to the laser plane will cause relative displacement of the plane. In small ring laser gyros, for instance, with a total path of less than 15 inches, the laser block can be made rigid enough to prevent excessive dislocation of the laser plane due to block distortions. In this case, the major cause of variations in the laser plane is the inability of the piezoelectric path length control mechanism to accurately displace the mirror only in the plane of the laser beams. Non-uniform conditions of the piezoelectric elements and inaccuracies in the dimensions of the fabricated parts of the mechanism cause the mirror to tilt when it is moved to adjust the path length and this tilt causes the location of the laser plane to change.
It is therefore an object of the present invention to provide precise path length control in a ring laser gyro while minimizing mirror tilt.
Yet another object of the invention is such path length control which has a simple mechanical configuration.